Ethylene glycol can be prepared by a variety of routes including from monosaccharides, e.g. sugars, via fermentation and hydrogenolysis processes, or by hydroformylation.
The fermentation route is a five-step process wherein glucose is fermented to ethanol and carbon dioxide, followed by conversion of ethanol to ethylene, ethylene to ethylene oxide and ethylene oxide to ethylene glycol. One disadvantage of this method is that per mole of glucose fermented, two moles of carbon dioxide are produced together with two moles of ethanol; this has the effect that a theoretical maximum of 67% of the carbon present in the glucose can be transformed to ethanol.
The hydrogenolysis route is a two-step process wherein glucose is reduced to sorbitol followed by hydrogenolysis of sorbitol to ethylene glycol, as illustrated by U.S. Pat. No. 6,297,409B1 and US 2008/0228014 A1. Significant quantities of propylene glycol, compared to ethylene glycol, are formed via the hydrogenolysis process. Additionally, the amount of catalyst used is significant and appears difficult to reactivate once spent. Furthermore, the byproducts formed, in particular butanediols, are difficult to separate from the desired product. In particular, the industrially favorable method of distillation for separation (purification) purposes is extremely difficult to apply as the byproducts have very similar boiling points to the final product, and the desired product may react further, as illustrated in US2014/0039224 A1 and U.S. Pat. No. 5,393,542 B1.
The hydroformylation route is a two-step route wherein glycolaldehyde is prepared from formaldehyde, carbon monoxide and hydrogen, followed by hydrogenation of the glycolaldehyde to ethylene glycol, as illustrated in U.S. Pat. No. 4,496,781 B1. There appears to be several extraction steps present in order to separate formaldehyde from glycolaldehyde and proceed with the hydrogenation reaction.
Therefore it is desirable to provide an alternative, improved, high yielding and industrially feasible process for the preparation of ethylene glycol from sugars. An additional advantage would be the use of greater than 67% of the sugar carbon atoms present in the final product or a commercially valuable byproduct.
It could be conceived that ethylene glycol may be prepared via a process comprising two steps; such as the preparation of glycolaldehyde from sugars and its subsequent hydrogenation to glycols. The two steps of the proposed processes appear to be independently successful as illustrated in the following paragraphs.
It is known that sugars may be pyrolysed to obtain a pyrolysis product composition comprising oxygenate compounds such as glycolaldehyde U.S. Pat. No. 7,094,932 B2; the pyrolysis product composition typically comprises C1-C3 oxygenate compounds, including formaldehyde, glycolaldehyde, glyoxal, pyruvaldehyde and acetol. The main product of this reaction is glycolaldehyde (U.S. Pat. No. 7,094,932 B2). Water is the solvent of the reaction.
It is also known that pure glycolaldehyde may be hydrogenated to ethylene glycol. U.S. Pat. No. 4,200,765 B1 discloses hydrogenation of glycolaldehyde under severe conditions: at high pressure [3000 psi (ca. 345 bar)], high temperature [150° C.], with an organic solvent [N-methyl pyrrolidine] and a palladium on carbon [Pd/C] catalyst for a prolonged period [5 h]. U.S. Pat. No. 4,321,414 B1 and U.S. Pat. No. 4,317,946 B1 disclose the hydrogenation of glycolaldehyde with a homogenous ruthenium catalyst and U.S. Pat. No. 4,496,781 B1 discloses a continuous flow hydrogenation at low pressure [500 psi (ca. 35 bar)], at high temperature [160° C.] with a ruthenium on carbon catalyst [Ru/C] in ethylene glycol and trace acetonitrile as solvent.
As illustrated, the two steps, pyrolysis of glucose to obtain, inter alia glycolaldehyde, and hydrogenation of pure glycolaldehyde, appear to be independently possible. However, in order for the pyrolysis product composition to be hydrogenated, laborious separation processes must be employed to remove formaldehyde from the pyrolysis product composition as formaldehyde is a known poison of hydrogenation catalysts U.S. Pat. No. 5,210,337 B1. U.S. Pat. No. 5,393,542 B1 discloses an exemplary purification process comprising multiple distillation steps followed by a solvent-induced precipitation to obtain glycolaldehyde. Therefore, it is not possible to hydrogenate the product of the pyrolysis step (the pyrolysis product composition) directly, as formaldehyde is present in the composition in a significant amount.
In addition to the requirement of removing formaldehyde, which would increase the number of process steps required, it would also be a great advantage industrially to use a solvent that is non-toxic, for example water. Therefore it would be a significant advantage to be able to carry out the hydrogenation step in the presence of formaldehyde, using a non-toxic solvent and in the solvent of the previous (pyrolysis) reaction.
With regard to hydrogenation of glycolaldehyde, although there is the provision of suitable reaction conditions to obtain a high yield in organic solvents, the reaction with water as a solvent appears to be less successful. U.S. Pat. No. 5,393,542 B1 discloses thermal degradation of glycolaldehyde (2-hydroxyacetaldehyde) when subjected to temperatures of 90° C. or higher and where water is the solvent.
EP 0 002 908 B1 discloses the variation in yield (conversion and selectivity) of the hydrogenation of glycolaldehyde reaction with the use of various catalysts in an aqueous solution at 110° C.: Raney Nickel [100% conversion 49.4% selectivity], 10% Pd/C [62% conversion, 61% selectivity] and 10% Pt/C [100% conversion, 73% selectivity]. An additional disadvantage of catalysts used in liquid water is the strain on the catalyst. In particular, at high temperatures (>160° C.) many supports are not stable and will dissolve, degrade or the surface area is reduced; Energy & Fuels 2006, 20, 2337-2343. Hence, special catalysts are needed and the long-term catalyst performance is often problematic, consequently, the catalyst must be replaced frequently (ca. 3-6 months). Consequently, mild reaction conditions are favorable in order to ensure longevity of the catalyst, in particular on an industrial scale and in industrial conditions.
In addition, the choice of catalyst may affect the decomposition of glycolaldehyde when in the presence of the catalyst; U.S. Pat. No. 5,210,337 B1 discloses the problem of glycolaldehyde ‘unzipping’ to form formaldehyde and consequently poisoning the hydrogenation catalyst. It is also possible that glycolaldehyde may self-condense or condense with another C1-C3 oxygenate compound, also illustrated in U.S. Pat. No. 5,210,337 B1. Additionally, the choice of catalyst and stability of the glycol product may affect the degree of reduction of the glycolaldehyde. It is possible that a catalyst may reduce the glycolaldehyde to ethanol or ethane, i.e. over reduce the glycolaldehyde.
Additionally, it is known that an increase in temperature, concentration of the substrate and amount and identity of catalyst present affects the yield (conversion and selectivity) of hydrogenation reactions of glycolaldehyde. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis, Shigeo Nishimura, ISBN: 978-0-471-39698-7, April 2001.
As demonstrated, an industrial scale process for preparing ethylene glycol from sugars via pyrolysis of sugars and subsequent hydrogenation is hindered from two perspectives. The first is the requirement to remove formaldehyde from the pyrolysis product composition in order to enable successful hydrogenation. The second is the provision of mild reaction conditions that are high yielding. Furthermore, in order to provide a process that is industrially and commercially feasible, it is desirable to provide a high yielding two-step process that utilizes non-toxic solvents and produces a minimal amount of byproducts that are separable from the ethylene glycol product using industrially feasible techniques. The ability to separate byproducts from the ethylene glycol product enables the ethylene glycol to be used in processes such as polymer production. Polymer production requires substrates to be in a highly pure form.
It has now been discovered that the pyrolysis product composition obtainable from the pyrolysis of sugars may be hydrogenated in a high yield, in mild conditions, with minimal byproducts and directly from a pyrolysis product composition comprising formaldehyde.